Conventional colorimeters measure the light reflected from a sample in several discrete wavebands in the visible range of the spectrum. For example, for 10 nanometer (nm) wide bands, there may be 41 discrete data points recorded from 380 to 780 nm. Each of these data points represents the reflectance of the sample for light in that band. These 41 data points are then processed to generate a numerical description of the chromaticity when viewed under a user-defined illuminant.
When fluorescent materials are illuminated with light of one waveband, however, light is reflected not only in that band, but light may also be emitted at different wavelengths, resulting from the ability of fluorescent materials to absorb light at one wavelength and emit fluorescent light at different, usually longer wavelengths. To measure the color of a fluorescent sample, therefore, many more data points are required, such as, for example, an additional 1148 data points for the same 10 nm bandwidth. These 1148 data points represent the light emitted in one band due to light absorbed in another (shorter wavelength) band. The excitation range, moreover, must be extended down to the lowest wavelength of expected illumination, generally 300 nm. These 1189 "bispectral" data points are then processed to generate a numerical description of the chromaticity as viewed under the user-defined illuminant, as shown in later-described FIG. 1--a grid showing the above-described 41 reflectance data points and the 1148 fluorescence or luminescence data points on an excitation-emission grid with the exemplary 10 nm bandwidth.
Bispectral fluorescence measuring colorimeters have heretofore been designed and custom-built on a one-of-a-kind basis by large laboratories for their own personal use, and have been large systems filling an entire room, highly expensive, very slow operating (up to 17 hours for a measurement in some cases), and difficult to service.
Among these systems is the spectrofluorimeter of the National Physical Laboratory of the United Kingdom described in a paper entitled "Fluorescent Standards For Surface Colour", published in August, 1995 as NPL Report QU111 and authored by D. C. Williams, and a different spectrofluorimeter described in a paper by Jim Leland and Angelo Arecchi; two of the inventors of the present patent application, entitled "Principles of Bispectral Fluorescence Colorimetry". Photometric Engineering of Sources and Systems, Angelo V. Arecchi, Editor, Proceedings of SPIE Vol. 3140, 76-87 (1997). Large laboratory fluorescent standard measuring colorimeters have also been constructed for government research use at other government laboratories--at BAM, Berlin, Germany, and at NRC, Ottawa, Canada, J. C. Zwinkels and D. S. Gignac, "Development of a new reference spectrofluorimeter in Spectrophotometry, Luminescence and Colour: Science and Compliance. C. Burgess and D. G. Jones, eds. (Elsevier, Amsterdam, 1995); and J. C. Zwinkels, D. S. Gignac, M. Nevins, I. Powell, and A Bewsher, "Design and testing of a two-monochromator reference spectrofluorimeter for high-accuracy total radiance factor measurements." Applied Optics. Vol. 36. No. 4,892 (1997).
Other references discussing the problems of spectrofluorimetric measurement include the following:
F. J. J. Clarke, "Problems of spectrofluorimetric standards for reflection and colorimetric use", NPL Report MOM 12, National Physical Laboratory Division of Mechanical and Optical Metrology, (NPL August 1975). PA1 R. Donaldson, "Spectrophotometry of fluorescent pigments", British Journal of Applied Physics, 5.210 (1954). PA1 F. Grum, "Colorimetry of fluorescent materials", in Color Measurement, F. Grum and C. J. Bartleston, eds., Vol. 2 of Optical Radiation Measurements (Academic Press, Inc. New York, 1983). PA1 D. Gundlach and H. Terstiege, "Problems in Measurement of Fluorescent Materials", Color Research and Applications, Volume 19, Number 6, (1994). PA1 K. D. Mielenz, "Photoluminescence Spectrometry", in Measurement of Photoluminescence, K. D. Milenz, ed., Vol. 3 of Optical Radiation Measurements (Academic Press, Inc., New York, 198)2. PA1 D. C. Williams, "Fluorescent Standards for Surface Color", NPL Report QU 111, National Physical Laboratory, Division of Quantum Metrology, (NPL, August 1995). PA1 ASTM Standards on Color and Appearance Measurement, 3rd Edition (ASTM, Philadelphia, 1991). PA1 CTE Technical Committee TC-1.2 The Spectroradiometric Measurement of Light Sources, Publication CIE NO. 73 (CTE, Paris, 1984). PA1 CTE Technical Committee TC-1.3, Colorimetry, 2nd Edition, Publication CTE No. 15.2 (CTE, Vienna, 1986).
The present invention, on the other hand, is concerned with the specific problems attendant upon providing a small, commercial bench-top bispectral fluorescence colorimeter ("BTH" in later-described FIGS. 3 and 4) of low cost, high measurement speed (as of the order of ten minutes or less), and practical serviceability, embodying multiple on-board processors operated under judicious choices between software and firmware, and using modular designs of mechanical, optical and electronic components and circuits that enable such size, cost and serviceability reductions. An early prototype, not however embodying the novel features of the present invention as herein claimed, is described in a preliminary brochure of Labsphere, Inc., the assignee of the present application, entitled "The BPC-450 Bispectral Fluorescence Colorimeter" and printed in December, 1997.